The G540 has four separate driver modules that plug into an optically isolated motherboard. The Xylotex has everything on one (bare) PCB, if one driver is faulty you throw the whole thing away.
The G540 has a maximum input voltage of 50V compared to the 24V of the Xylotex.
The G540 includes two high current outputs that can drive relays, four isolated inputs, charge pump circuit; Xyloex has none of these features.
The G40 uses step morphing, at low RPMs you get all the smoothness of 10x micro-stepping, as the RPMs increase it morphs to full steps so you get full power from your motor; Xylotex does not do this.
The G540 is very hard to kill...
Not to sound like a fan-boy but you will spend more money building something equivalent to the G540, I suspect that is why they have sold tens of thousands of them.